Gas-lift system and method

ABSTRACT

A gas-lift system is provided with a multiple valve arrangement, wherein a plurality of gas-lift valves which are each actuated by a pressure-responsive member, are installed in the tubing string at each lifting depth in the borehole wherein only a single valve is conventionally mounted. Thus, each of the valves will independently respond to the same injection pressure, and will thereby independently perform the same function in the system as the other valves at each lifting depth. Each such valve is preferably provided with suitable fail/safe means, such as a spring, whereby the valve will automatically close in the event of malfunction, and each valve is also preferably provided with a dual shutoff assembly to guarantee absolute fluidtight closure in the event of failure.

United States Patent [72} inventors Everett D. McMurry;

Bolling A. Abercrombie, both of Houston, Tex. [21 1 Appl. No. 20,923 [22] Filed Mar. 19, 1970 [45] Patented Aug. 24, 1971 [73] Assignee McMurray Oil Tool S, Inc.

Houston, Tex.

[54] GASLIFI SYSTEM AND METHOD 14 Claims, 4 Drawing Fig.

52] us. C1. 166/224, 417/111, 417/115 [51] Int. CL, E211) 33/00 [50] Field of Search 166/224; 417/108,1l1,l12,l44,1l6,l15;137/516.25, 516.27, 516.29, 210

[56] References Cited UNITED STATES PATENTS 2,179,033 11/1939 Cashion etal. 166/224 Primary Examiner-James A. Leppink AttorneyAmold, White & Durkee ABSTRACT: A gas-lift system is provided with a multiple valve arrangement, wherein a plurality of gas-lift valves which are each actuated by a pressure-responsive member, are installed in the tubing string at each lifting depth in the borehole wherein only a single valve is conventionally mounted. Thus, each of the valves will independently respond to the same injection pressure, and will thereby independently perform the same function in the system as the other valves at each lifting depth. Each such valve is preferably provided with suitable fail/safe means, such as a spring, whereby the valve will automatically close in the event of malfunction, and each valve is also preferably provided with a dual shutoff assembly to guarantee absolute fluidtight closure in the event of failure.

IN VE N TORS Everett D. McMurry Bolling AA/Jercrombie M, WM & Owl/bee ATTORNEYS PATENTEI] M1824 I97l FIG. IA

GAS-LIFT SYSTEM AND METHOD BACKGROUND OF INVENTION This invention relates to gas-injection methods and apparatus for recovering oil from a subsurface earth formation, and more particularly relates to improved gas-lift methods and apparatus.

It is well known to inject gas into the casing in an oil well for the purpose of opening a gas-lift valve, whereby gas will enter the tubing stringand carry oil therein to the surface. It is ordinarily impossible to achieve sufficient lift with only a single valve assembly, and thus it is common practice to mount several valves on the tubing string in a spaced-apart manner. at different preselected depths in order to obtain an optimum rate of productivity from the well.

It is extremely inconvenient for one or more gas-lift valves to fail after it has been installed inthe well, since it is usually necessary to remove and replace a defective valve whenever failure occurs. Such valves are relatively expensive. The real expense, however, is the cost of removing and replacing the valve, since elaborate wire line retrievaltechniques must be used which are often time-consuming as well as expensive. Moreover, in those systems wherein the defective valve is mounted in a conventional mandrel, it is impossible to utilize wire line retrieval techniques and it is necessary to pull the entire tubing string out of the casing merely to remove and replace the valve.

The problem of valve failure has previously been attacked merely by efforts to improve the reliability of the equipment, and substantial progress has indeed been made. Nevertheless, even a valve of the type depicted in US. Pat. No. 3,342,203 is subject to an occasional failure, and when this happens in any of the gas-lift systems now in operation, the valve must be removed and replaced.

These disadvantages of the prior art are overcome with the present invention, and novel gas-lift methods and apparatus sity to remove or replace a defective gas-lift valve.

SUMMARY OF INVENTION In a preferred embodiment of the present invention, two or more gas-lift valves are located at each of the aforementioned depths instead of a single gas-lift valve as hereinbefore described. Each of the valves is adapted to perform the same function, however, and thus such valves are located as near as possible at the same depth in the borehole.

Since all of the valves in each pair or set" are located at substantially the same depth in theborehole so as to perform the same function in the system, all valves in a set will necessarily be set to respond to the same preselected casing pressure. It is preferable that all valves in the set are mounted on lugs spaced circumferentially about the mandrel, whereby all valves in the set will be located at exactly the same depth in the well and subject to exactly the same casing pressure. In most cases, however, the well casing will be too small to receive such a mandrel. Furthermore, the well bore pressure differential over a few feet of borehole depth is relatively minor. Accordingly, it has been found satisfactory for the purposes of the present invention to arrange thevalves in the set one immediately above or below the other, provided all lugs on the mandrel are spaced as close together as practical. In all other respects, the mandrel may be of conventional design.

Any gas-lift valve of any design may fail because of any number of different reasons. In most cases, however, the source of the failure is related more or less directly to the diaphragm or bellows which operates to keep the valve closed except when the casing pressure at that depth is sufficient to compress the bellows and open the valve. Accordingly, in most instances of failure the valve will stand open, irrespective of the casing pressure.

terconnected to close the valve in the event of failure of the bellows or diaphragm. Thus, such a valve will normally open only when the casing pressure is sufficient to overcome the combined force of both the dome pressure in the bellows and the force of the spring.

On the other hand, if the bellows (or diaphragm) becomes damaged whereby the pressure-in the bellows is no longer isolated from the casing pressure, the internal pressure in the bellows will always equal the external pressure, no matter what the casing pressure may be. Whenever this happens, the only force being applied to. the shutoff body or component in the valve will be that of the fail/safe spring, and thus the spring will then slam the valve shut.

Any of various types of gas-lift valves may be employed to obtain the benefits of the instant invention, provided both valves in each pair are equipped with a fail/safe spring or other closing device as hereinbefore described. It is well known,

toff in the event that the other is incapable of interrupting fluid flow through the valve. In the preferred embodiment of the invention, the secondary shutoff member may be located downstream of the primary shutoff member so as to seat before the primary shutoff member. Thus, the secondary shutoff member will be the member which is principally subject to damage from high velocity fluid flow across its surfaces, and

In the present invention, therefore, each gas-lift valve is pro- A the primary shutoff member will be protected from such damage since fluid flow across it will be interrupted or greatly reduced before it moves into engagement with the valve seat.

More particularly, the secondary or downstream shutoff member is preferablylslidably mounted on or interconnected with the primary shutoff member. Although both shutoff members are connected to be driven into the valve seat by the fail/safe spring, the secondary member will seat-first before the primary member is seated. If the primary shutoff member achieves complete shutoff, the position of the secondary shutoff will then be immaterial. On'the other hand, if gas continues to bleed across the primary shutoff member, the pressure differential across the secondary member will then force it into fluidtight engagement with the valve seat to provide complete interruption of fluid flow through the valve.

In thepreferred embodiment of the present invention, each of the plurality of gas-lift valves located ateach of the various lifting depths which have been selected, is'also provided with at least one and preferably two check valves positioned downstream of the shutoff members and closable in response to excessive tubingpressures to prevent backflow into the eas- It will be apparent from the foregoing that merely adding a single backup valve will substantially increase the odds against failure of the overall system as a result of malfunction of one of the valves. Accordingly, this alone greatly reduces the risk that wire line retrieval or removal of the tubing string will ever be required during the life of the average well. The advantage of the backup valve is greatly reduced, however, if the defective valve stands open in the event of failure. Thus, the fail/safe spring or other device is an important feature of the present invention, since the defective valve cannot be allowed to remain in the tubing string indefinitely unless the defective valve is adapted to close whenever it fails.

These and other features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawing.

DRAWING FIGS. 1A and 1B provide a pictorial representation, partly in cross section, of the upper and lower sections, respectively,

of an exemplary form of gas-lift valve apparatus suitable for purposes of the present invention. I

FIG. 2 provides a simplified functional representation of a plurality of such gas-lift valves mounted in typical borehole in a manner contemplated by the concept of the present inventron.

FIG. 3 provides a pictorial representation of one type of mandrel suitable for supporting gas-lift valves of the type depicted in FIG. 1.

DETAILED DESCRIPTION f 12 of conventional design may be installed immediately above the lower end of the tubing string 5 to route fluid flow from the formation 13 into the tubing string 5. In addition, and in accordance with the concept of the present invention, a plurality of gas-lift valves 6-11 are preferably installed in the tubing string 5 in pairs at various preselected depths along the borehole 2. Thus, gas-lift valves 6 and 7 will each operate to .perform the same function in the system, since both are located at substantially the same depth in the borehole. Similarly, the pair of gas-lift valves 8 and 9 located at another lower depth in the borehole 2 will both perform the same function in the system, and the lowest pair of gas-lift valves 10 and 11 will both perform'the same function.

The gas-lift system depicted generally in FIG. 2 may be operated in a conventional manner, wherein gas which is injected into the casing 4 from the surface may be routed into the tubing string 5 through the various gas-lift valves 6-11 whenever the pressure in the casing 4 is sufficient to open the valves 6-11. The casing pressure will exceed the tubing pressure, and thus gas entering the tubing string 5 will expand and carry liquid from the formation 13 to the surface.

The difference in depth between pluralities of gas-lift valves which arearranged in such a manner is relatively minor insofar as their actuating pressures are concerned.

Referring now to FIG. 3,. there may be seen a pictorial representation of an alternative form of mandrel 15 which is suitable for the purposes of the present invention, and which .maybe used in place of the more conventional type of assembly suggested in H0. 2. In particular, the mandrel 15 may be seen to have pressure connections 16 and 17, which are otherwise of generally conventional design, but which are located on opposite sides from each other so as to support gaslift valves 18 and 19 at precisely the same lifting depth in the well.

Referring now to FIGS. 1A and 18, there may be seen a pictorial representation, partly in cross section, of a gas-lift valve assembly which is particularly suitable for the purposes of the present invention. As will also be apparent, the upper section or portion of the assembly is depicted in FIG. 1A, and the lower'portion of the assembly is depicted in FIG. 1B.

Referring first to FIG. 13, there may be seen a cylindrical or annular valve seat body 20 supported in a tubular seat holder section 21 by means of a snap ring 22 or other conventional device. A tubelike valve body section 23 is threadably attached at one end to the seat holder section 21, and is provided with a pair of oppositely positioned intake ports 24 adjacent or next to the valve seat body 20, and may also have another pair of oppositely positioned ports 40 adjacent its opposite or other end. Referring again to F IO. 18, the valve seat body 20 may be an annular member having an inwardly tapering throat 25A which connects downstream with a flow passage 25B, and which is adapted to be closed by a secondary shutoff member 26 having a sealing head portion 27 and a cylindrical shank portion 28. The principal or primary shutoff member 31 may be positioned upstream of the secondary shutoff member 26, and may be supported in concentric or coaxial alignment with the valve seat body 20 by a shutofi drive member 33 having a support portion 34 threadably connected to the primary shutoff member 31, and a cylindrical guide por' tion 35 slidably inserted in the adjacent end of a supporting tube or sleeve 36.

Referring again to the structure illustrated in FIG. 1B, the shank 28 of the secondary shutoff member 26 may be slidably inserted concentrically in the end of the primary shutoff member 31 in confronting alignment with the axis of the valve seat body 20 whereby its head 27 will be thrust into the throat 25A of the valve seat body 20. In addition, the primary shutoff member 31 may be provided with a transverse pin aperture 32 to loosely accommodate the extended ends of a pin which is fixedly inserted through the end of the shank portion 28 of the secondary shutoff member 26..Thus, the secondary shutoff member 26 will be slidably movable to and from the primary shutoff will be effected whenever the head portion 27 of the secondary shutoff member 26 is driven into sealing engage- I ment with the surface of the throat 25A of the valve seat body 20. If the secondary shutoff member 26 fails to completely stopper the throat 25A in the valve seat body 20, the primary shutoff member 31 will thereafter move into sealing engagement with the valve seat body 20 as indicated in FIG. 18.

It will be apparent that, in the apparatus depicted in FIG. 13, both shutoff members 26 and 31 are driven into engagement with the valve seat body 20 by the bellows 37 and the fail/safe spring 42 hereinafter described. Although it is the secondary shutoff member 26 which will seat first, it is expected and desired that fluidtight engagement will be effected by either component. However, if ,the gas-lift valve assembly has been in service for an extended period, it may also be expected that bits of rock and sand will have unavoidably scored and damaged the sealing head 27 of the secondary shutoff member 26 and the throat 25A of the valve seat body 20 to an extent such that complete shutoff cannot be achieved with these components. The fail/safe spring 42 will drive the primary shutoff described. 31 into engagement with the valve seat body 20, however, irrespective of whether the secondary shutoff member 26 has completely interrupted fluid flow through the flow passage 258. Thus, complete shutoff will be substantially assured with such an arrangement inasmuch as the odds against failure of both shutoff members 26 and 31 are almost astronomical.

It should also be noted that although shutoff members 26 and 31 both engage the valve seat body 20, member 26 engages a different part of seat body 20 than does member 31. More particularly, the sealing head 27 of the secondary shutoff member 26 is thrust into the throat 25A of the valve seat body 20, whereas the primary shutoff member 31 is urged against the annular rim 25C portion of the valve seat body 20. Accordingly, it will be apparent that erosion of the shutoff surfaces of the seat body 20 will occur primarily with respect to he surface of the throat 25A, rather than to the rim 25C of the seat body 20. Further, it is the head 27 of the shutoff member 26, rather than the primary shutoff member 31, which will tend to be eroded.

Referring again to FIG. 18, there is illustrated apparatus which may be seen to further include a metal bellows 37 of generally conventional configuration mounted within the valve body section 23 and interconnected with and between the shutoff drive member 33 and the fail/safe body section 41 which is threadably attached to the other end-of the tubular valve body section 23. The bellows 37 is preferably provided with a plurality of both inside support rings 38 and outside support rings 39 as more particularly depicted and described in the copending patent application Serial No. 696,456, filed Jan. 8, 1968, by E. D. McMurry, B. A. Abercrombie and K. I. Bruun.

A bellows 37 of conventional design will normally contain an internal or dome" pressure which is normally greater than ambient pressure, and which will therefore tend to longitudinally distend the bellows 37 to cause it to drive the shutoff members 26 and 31 into engagement with the valve seat body 20. When the ambient pressure exceeds the dome pressure, however, the bellows 37 will be longitudinally compressed and this will pull the shutoff members 26 and 31 free of the valve seat body 20, whereby fluid may flow through the intake port 24 and throat 25A and into the flow passage 258.

It will be apparent that, but for the fail/safe spring 42, movement of the shutoff members 26 and 31 to and from the valve seat body depends on the existence of a differential between he internal and external pressures of the bellows 37. Thus, if a leak develops in the wall of the bellows 37 whereby the internal and external pressures are equalized, the bellows 37 will not be longitudinally distended no matter what the ambient or external pressure may be, and the valve will tend to stand open at all times.

As hereinbefore stated, it is a feature of the present invention that fail/safe means be included for the purpose of shuting the valve in the event of failure of the apparatus. In the assembly depicted in FIG. 1B, the fail/safe function is provided by the helical spring 42 which is disposed about he sleeve 36 in a partially compressed manner between a movable spring guide member 43 and an end fitting 44 which is treadedly connected in a fluidtight manner to the opposite end of a tubular fail/safe body sectionor housing 41. Thus, be compressed spring 42 functions to constantly urge the guide member 43 against an external shoulder portion of the sleeve '36, and this,

in turn, serves to drive the sleeve 36, the shutoff drive member 33, and both shutofi members 26 and 31 toward and against the valve seat body 20. i

The primary closing force which is applied to the primary shutoff member 31 is applied by the bellows 37 rather than the spring 42. However, if the casing pressure entering the ports 40 rises above the dome pressure in the bellows 37, the bellows 37 will longitudinally contract, and will override the spring 42 and draw the primary shutoff member 31 away from engagement with the valve seat body 20. Since the secondary shutoff member 26 will also stopper the throat A, contraction of the bellows 37 must be great enough to also carry the heat 27 of the secondary shutoff member 26 out of the throat 25A and away from the valve seat body 20 before the gas will flow from the casing 4 to the tubing string by way of the intake ports 24 and the flow passage 25.

The end fitting 44 is preferably of conventional design, and as indicated in FIG. 18, may include a suitable fill connection for injecting the dome pressure into the bellows 37. Thus, the bellows 37 is preferably gas-tightly secured at one end to the threadably interconnected at its opposite end to another check valve body 47. An O-ring 46 or other suitable sealing device may be provided for preventing fluid flow from the'casing 4 into the interior of the check valve body 47, and a check member 48 of suitable configuration may be arranged within the check valve body 57 for stoppering the passageway leading from the valve seat holder section 21 though the check valve seat body 45.

The check member 48 may be spring-loaded in a conventional manner for fluidtight engagement with the check valve seat body 45. However, this is only whenever the pressure within the tubing string 5 exceeds the pressure within the casing 4, and, as suggested in FIG. 1A, such spring-loading is not essential and may be omitted.

Referring again to FIG. 1A, there may be seen a second such check valve assembly arranged downstream of the check valve assembly hereinbefore described. In particular, the opposite end of the first check valve body 47 may be threadably interconnected with the upstream end of a second check valve body 50, and another check member 51 may be positioned i the check valve body 50.

shutoff drive member 33, and at the other end to the upper end of the fail/safe body section or housing 41. Accordingly, the dome pressure will be contained in a reservoir which is composed of the end fitting 44, the fail/safe body section or housing 41 and the bellows 37.

If a leak develops which results in a loss of dome pressure, or if the wall of the bellows 37 develops a lead, this will equalize the pressure in the bellows 37 with the ambient pressure entering through ports 40, and the bellows 37 will no longer drive the secondary shutoff member 31 toward the valve seat body 20. In such a case, there would be fluid flow between the casing 4 and tubing string 5, except that the spring 42 will now drive both shutoff members 26 and 31 into shutoff engagement with the valve seat body 20.

Referring now to FIG. 1A, it will be seen that the upper end of the valve seat holder section 21 may b e threadably interconnected to one end of a check valve seat body 45 which is It will be apparent that many modifications and alterations may be made in the structures and techniques hereinbefore described, without necessarily departing substantially from the concept of the present invention. For example, it is frequently desirable to produce the oil through the casing 4 instead of through the tubing string 5, and to inject gas down through the tubing string 5 instead. If this is desired, the gaslift valves 6-11 will preferably be reversed in order to respond to tubing pressure rather than to casing pressure, and injection gas will be supplied through the tubing string 5 instead of through the casing 4 as hereinbefore described. In such an arrangement, the apparatus depicted in FIGS. 1A and IE will operate substantially the same as before, however.

It should also be noted that it is often desirable to provide three or more valves at each lifting depth in the borehole. According, the system depicted in FIG. 2 wherein only two valves are positioned at each lifting depth in the borehole, is merely intended to illustrate the concept of the invention, and is not intended to limit the invention to only two identicalvalves in each set.

Other modifications and variations will become apparent from the structures and techniques hereinbefore described. Accordingly, it should be clearly understood that the apparatus and methods illustrated in the accompanying drawing and referred to in the foregoing description are intended st be exemplary only, and are not intended to be limitations on the concept of the present invention.

What we claim is:

1. A gas-lift system for use in a borehole containing a production conduit and a gas injection conduit, comprising a plurality of gas-lift valves each separatelyinterconnected between said conduits at generally the same depth in said borehole for similarly but independently and automatically admitting gas into said production conduit from said injection conduit, said gas-lift valves being capable of simultaneously admitting gas into said production conduit from said injection conduit,

each of said valves further comprising independent means biasing said closure means toward the closed position thereof and a pressure-responsive member for overriding said biasing means and independently opening each of said valves.

2. The system described in claim 1, wherein said pressureresponsive member in each of said plurality of valves at said same borehole depth is responsive to substantially the same ambient pressure to open and close its respective valve,

operable independently and separately of the other pressure-responsive members in said plurality of valves, and

wherein said biasing means in each of said plurality of valves is operable separately and independently of the operation of the biasing means and pressure-responsive members in the'other of said plurality of valves.

3. In a gas-lift system adapted to produce oil or the like through a borehole containing a production conduit and an injection conduit,

the improvement in combination therewith comprising a mandrel adapted to be interconnected with said production conduit,

a first gas-lift valve assembly interconnected with said mandrel at substantially a preselected depth in said borehole and having a plurality of shutoff members operable in response to apreselected pressure in said borehole which is functionally related to said preselected borehole depth,

a second gas-lift valve assembly interconnected with said mandrel at substantially the same depth in said borehole as said first valve assembly and having a plurality of shutoff members operable substantially in response to said preselected pressure in said borehole and independently of said shutoff members in said first valve assembly.

4. The apparatus described in claim 3, wherein said gas-lift valve assemblies each further comprise pressure-responsive means for actuating said pair of shutoff members in response to said preselected borehole pressure and independently of the other valve assembly, and

biasing means for actuating said shutoff members and yieldable to said pressure-responsive means associated therewith.

5. The apparatus described in claim 4, wherein each of said gas-lift valve assemblies further comprises shutofi' engagement with said valve seat body by said pressure-responsive means.

6. The apparatus described in claim 5, wherein the biasing means in each of said gas-lift valves includes a spring means urging said shutoff members into shutoff engagement with said valve seat body and yieldable to actuation of said pressure-responsive means.

7. The apparatus described in claim 6, wherein said first shutoff member is arranged to follow said second shutoff member into engagement with said valve seat body.

8. A gas-lift system for use in a borehole containing a production conduit and a gas injection conduit, comprising a plurality of gas-lift valves each separately interconnected between said conduits at generally the same depth in said borehole for similarly but independently admitting gas into said production conduit from said injection conduit, each of said gas-lift valves comprising a valve seat body,

a first shutoff member interconnected with said pressureresponsive member therein to be urged into shutoff engagement with said valve seat body, and

a second shutoff member interconnected between said first shutoff member and said valve seat body for shutoff en-I gagement with said valve seat body,

independent means biasing said closure means toward the closed position thereof,

a pressure-responsive member for overriding said biasing means and independently opening each of said valves,

said pressure-responsive member in each of said plurality of valves at said same borehole depth being responsive to substantially the same ambient pressure to open and close its respective valve, each of said pressure-responsive members also being operable independently and separately of the other pressureresponsive members in said plurality of valves,

said biasing means in each of said plurality of valves being operable separately and independently of the operation of the biasing means and pressure-responsive members in the other of said plurality of valves.

9. The system described in claim 8, wherein the first shutoff member in each of said valves follows said second shutoff member into shutoff engagement with said valve seat body.

10. The system described in claim 9, wherein the second shutoff member in each of said valves is slidably interconnected with said first shutoff member and movable independently of said first shutoff member through a predetermined distance between said valve seat body and said first shutoff member.

11. The system described in claim 10, wherein the biasing means in each of said valves includes a spring means interconnected to urge said first and second shutoff members into shutoff engagement with said valve seat body therein and yieldable to said pressure-responsive member associated therewith.

12. The system described in claim 11, wherein said pressure-responsive member includes a bellows assembly having a preselected internal dome pressure and arranged to urge said shutoff members into shutoff engagement with said valve seat body,

said bellows assembly being further arranged to override said spring means and to draw said shutoff members out of shutoff engagement with said valve seat body in response to an ambient pressure grater than said internal dome pressure. i

13. The system described in claim 12, wherein each of said assemblies further comprises check means positioned downstream of said valve seat body for interrupting fluid flow therethrough in response toa pressure in said production conduit greater than the pressure in said injection conduit.

14. The check means described in claim 13, further comprising a first check valve positioneddownstream of said valve seat body and closable in response to pressure in said production conduit greater than pressure in said injection conduit, and second check valve positioned between said valve seat body and said first check valve and closable in response to pressure in said production conduit greater than pressure in said injection conduit to establish a pressuretrapping region across said valve seat body wherein the pressure is less than either said pressure in said injection conduit or said pressure in said production conduit.

9 55 UNITED s'm'ncs m'ncw'r 01 1 111111 CERTIFICATE OF CORRECTION Patent: No. 3,601,191 Dated August 2 4, 1971 Inventor(s) Everett D. McMurry and Bolling A. Abercrombie It is certified that error appears in the above-identified patent and that: said Letters Patent are hereby corrected as shown below:

Abstract, line 20, "fluidtight" should read --fluidtig;ht; Column 2, line l6, "fluidtight" should read fluid-tig;ht-; Column 3, line 1, before "typical" insert a; Column 3,

line 67, "tubelike" should read -tubel ike; Column 4,

line U9, "described" should read member Column 4, line 66, "he" should read the; Column 5, line 21, "he" should read the-; Column 5, line 31, '"he" .should read the; Column 5, line 35, 'he" should read the; Column 5, line 50, "heat" should read head; Column 5, line 52, before "by" insert 5; Column 5, line 65, "lead" should read -leak;

Column 6, line 6, "57" should read I7-; Column 6, line 45, "cording," should read -cordingly,; Column 6, line 5 4,

"st" should read -to; Column 8, line 42, "grater" should read --p;reater.

Signed and sealed this 7th day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHJ:JP.,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. A gas-lift system for use in a borehole containing a production conduit and a gas injection conduit, comprising a plurality of gas-lift valves each separately interconnected between said conduits at generally the same depth in said borehole for similarly but independently and automatically admitting gas into said production conduit from said injection conduit, said gas-lift valves being capable of simultaneously admitting gas into said production conduit from said injection conduit, each of said valves further comprising independent means biasing said closure means toward the closed position thereof and a pressure-responsive member for overriding said biasing means and independently opening each of said valves.
 2. The system described in claim 1, wherein said pressure-responsive member in each of said plurality of valves at said same borehole depth is responsive to substantially the same ambient pressure to open and close its respective valve, wherein each of said pressure-responsive members is also operable independently and separately of the other pressure-responsive members in said plurality of valves, and wherein said biasing means in each of said plurality of valves is operable separately and independently of the operation of the biasing means and pressure-responsive members in the other of said plurality of valves.
 3. In a gas-lift system adapted to produce oil or the like through a borehole containing a production conduit and an injection conduit, the improvement in combination therewith comprising a mandrel adapted to be interconnected with said production conduit, a first gas-lift valve assembly interconnected with said mandrel at substantially a preselected depth in said borehole and having a plurality of shutoff members operable in response to a preselected pressure in said borehole which is functionally related to said preselected borehole depth, a second gas-lift valve assembly interconnected with said mandrel at substantially the same depth in said borehole as said first valve assembly and having a plurality of shutoff members operable substantially in response to said preselected pressure in said borehole and independently of said shutoff members in said first valve assembly.
 4. The apparatus described in claim 3, wherein said gas-lift valve assemblies each further comprise pressure-responsive means for actuating said pair of shutoff members in response to said preselected borehole pressure and independently of the other valve assembly, and biasing means for actuaTing said shutoff members and yieldable to said pressure-responsive means associated therewith.
 5. The apparatus described in claim 4, wherein each of said gas-lift valve assemblies further comprises a valve seat body, a first shutoff member interconnected between said pressure-responsive means and said valve seat body to be urged into shutoff engagement with said valve seat body by said pressure-responsive means, and a second shutoff member positioned between said first shutoff member and said valve seat body and to be urged into shutoff engagement with said valve seat body by said pressure-responsive means.
 6. The apparatus described in claim 5, wherein the biasing means in each of said gas-lift valves includes a spring means urging said shutoff members into shutoff engagement with said valve seat body and yieldable to actuation of said pressure-responsive means.
 7. The apparatus described in claim 6, wherein said first shutoff member is arranged to follow said second shutoff member into engagement with said valve seat body.
 8. A gas-lift system for use in a borehole containing a production conduit and a gas injection conduit, comprising a plurality of gas-lift valves each separately interconnected between said conduits at generally the same depth in said borehole for similarly but independently admitting gas into said production conduit from said injection conduit, each of said gas-lift valves comprising a valve seat body, a first shutoff member interconnected with said pressure-responsive member therein to be urged into shutoff engagement with said valve seat body, and a second shutoff member interconnected between said first shutoff member and said valve seat body for shutoff engagement with said valve seat body, independent means biasing said closure means toward the closed position thereof, a pressure-responsive member for overriding said biasing means and independently opening each of said valves, said pressure-responsive member in each of said plurality of valves at said same borehole depth being responsive to substantially the same ambient pressure to open and close its respective valve, each of said pressure-responsive members also being operable independently and separately of the other pressure-responsive members in said plurality of valves, said biasing means in each of said plurality of valves being operable separately and independently of the operation of the biasing means and pressure-responsive members in the other of said plurality of valves.
 9. The system described in claim 8, wherein the first shutoff member in each of said valves follows said second shutoff member into shutoff engagement with said valve seat body.
 10. The system described in claim 9, wherein the second shutoff member in each of said valves is slidably interconnected with said first shutoff member and movable independently of said first shutoff member through a predetermined distance between said valve seat body and said first shutoff member.
 11. The system described in claim 10, wherein the biasing means in each of said valves includes a spring means interconnected to urge said first and second shutoff members into shutoff engagement with said valve seat body therein and yieldable to said pressure-responsive member associated therewith.
 12. The system described in claim 11, wherein said pressure-responsive member includes a bellows assembly having a preselected internal dome pressure and arranged to urge said shutoff members into shutoff engagement with said valve seat body, said bellows assembly being further arranged to override said spring means and to draw said shutoff members out of shutoff engagement with said valve seat body in response to an ambient pressure grater than said internal dome pressure.
 13. The system described in claim 12, wherein each of said assemblies further comprises check means positioned downstream of said valve seat body fOr interrupting fluid flow therethrough in response to a pressure in said production conduit greater than the pressure in said injection conduit.
 14. The check means described in claim 13, further comprising a first check valve positioned downstream of said valve seat body and closable in response to pressure in said production conduit greater than pressure in said injection conduit, and a second check valve positioned between said valve seat body and said first check valve and closable in response to pressure in said production conduit greater than pressure in said injection conduit to establish a pressure-trapping region across said valve seat body wherein the pressure is less than either said pressure in said injection conduit or said pressure in said production conduit. 